This invention relates to the purification of zinc sulphate solution and, in particular, relates to the removal of chloride impurity from aqueous solutions of zinc sulphate intended for use in the electrolytic recovery of zinc.
The presence of chloride in zinc sulphate solution, which may be introduced into the solution during hydrometallurgical treatment of zinc sulphide concentrate, e.g., during pressure leaching with sulphuric acid, has a deleterious effect on electrodes during electrolysis. For example, leaching of a typical Mississippi Valley-type concentrate containing 55% Zn and 0.08% Cl in a single pass of return acid from zinc electrolysis may dissolve about 145 mg/L chloride. To avoid deleterious effects of chloride during electrolysis, this chloride should not exceed 100 mg/L. Use of return acid initially containing 100 mg/L chloride would produce a leach solution containing about 245 mg/L in the next pass and, without chloride removal, the amount of chloride in solution would increase as cycling of return acid continues. It is therefore evident that a process is required in which all the solution being used in electrolysis is treated to decrease the chloride to not more than 100 mg/L or a portion of the electrolyte is treated for substantially complete removal of chloride and then mixed with untreated solution. The latter is considered to be more economic.
It is noted in U.S. Pat. No. 1,403,065 that corrosion of electrodes is largely reduced if chloride is removed from the electrolyte prior to the electrolysis of zinc-bearing solution. A method of chloride removal is provided wherein a portion of solution is withdrawn from the circuit, treated with a soluble silver salt to precipitate silver chloride, and purified solution is returned to the circuit. The silver chloride is reduced to metallic silver and reconverted into a soluble salt for re-use. In more detail, the portion of zinc-bearing solution to be treated is made slightly acid with sulphuric acid and finely powdered silver sulphate is added. The mixture is agitated for about one hour and then rendered neutral or slightly basic to coagulate silver chloride. The precipitate is allowed to settle and clear solution is decanted and treated with zinc dust to remove traces of silver. Sulphuric acid and zinc dust are added to bottom solution and silver chloride filter cake to reduce silver chloride to metallic silver. When reduction is complete, the precipitate of metallic silver is agitated for some time in slightly acid solution to ensure complete dissolving of excess zinc dust. The silver precipitate is washed with water until free of chloride and then dried and heated with pure sulphuric acid to 250.degree.-300.degree. C. to convert all the silver to sulphate. This is ground to a fine powder for re-use. Each cycle takes nearly 3 days to complete and, in an ongoing operation, a large inventory of costly silver is required. In 1936 AIME Transactions 121, pages 503-4, silver recoveries by this method are reported to be 97.5 to 98 percent per cycle, i.e., silver losses are about 2 percent per cycle.